Technical Field
[0001] The present invention relates to a double-sided friction stir welding method including:
disposing a pair of opposed rotating tools on the top and bottom sides of a butt joint
or a lap joint, which is a welded joint between metal sheets, so as to face each other;
moving the pair of rotating tools along the butt joint or the lap joint in a welding
direction while the pair of opposed rotating tools are rotated to thereby soften a
portion of the metal sheets by heat of friction between the rotating tools and the
metal sheets; stirring the softened portion with the rotating tools to generate plastic
flow to thereby join the metal sheets together. The invention also relates to a double-sided
friction stir welding device for double-sided friction stir welding.
[0002] The invention aims to solve a problem that may occur when the friction stir welding
method is applied to welding of metal sheets or when the friction stir welding device
is used, i.e., to eliminate poor plastic flow that occurs locally at the joint because
of the differences in temperature and plastic flow in the direction of the thickness
of the metal sheets at the joint to thereby eliminate welding defects advantageously,
in order to obtain sufficient strength, improve welding workability, and particularly
to improve the welding speed.
[0003] In the following description, a butt joint (or a lap joint) in which metal sheets
(e.g., steel sheets) are butted (or lapped) together but still unwelded is referred
to as an "unwelded joint," and an integrated portion joined by plastic flow is referred
to as a "welded joint."
Background Art
[0004] Patent Literature 1 discloses a friction welding method including: rotating one or
both of a pair of metal materials to generate frictional heat in the metal materials
to thereby soften the metal materials; and stirring the softened potion to cause plastic
flow to thereby join the metal materials together. However, in this technique, since
the materials to be joined are rotated, the shape and size of the metal materials
are limited.
[0005] Patent Literature 2 proposes a method for joining metal sheets continuously in their
longitudinal direction. In this method, a rotating tool formed of a material substantially
harder than the metal sheets is inserted into an unwelded joint between the metal
sheets. The rotating tool is moved while rotated to generate heat and plastic flow
between the rotating tool and the metal sheets to thereby join the metal sheets together
continuously in their longitudinal direction. In this technique, with the metal sheets
fixed, the rotating tool is moved while rotated to thereby join the metal sheets together.
One advantage is that substantially infinitely long members extending in their welding
direction can be welded by solid-state joining continuously in their longitudinal
direction. This technique further has the following various advantages. Since the
metal sheets are welded by solid-state joining using the plastic flow of the metal
caused by frictional heat generated by the rotating tool and the metal sheets, the
metal sheets can be joined without melting the joint. Since the heating temperature
is low, deformation after welding is small. Since the joint is not melted, less defects
are generated. Moreover, no filler metal is necessary.
[0006] The friction stir welding method has been increasingly used as a method for welding
low-melting point metal sheets typified by aluminum alloy sheets and magnesium alloy
sheets in the fields of aircraft, ships, railroad cars, automobiles, etc. The reason
for this is as follows. With a conventional arc welding method, it is difficult for
the welded joint between these low-melting point metal sheets to have satisfactory
characteristics. However, when the friction stir welding method is applied, productivity
can be improved, and the welded joint obtained can have high quality.
[0007] When the friction stir welding method is applied to structural steel sheets used
mainly for the materials of structures such as buildings, ships, heavy machines, pipelines,
and automobiles, solidification cracking and hydrogen cracking, which are problems
in the conventional fusion welding, can be avoided, and a structural change of the
steel sheets is prevented, so that an improvement in joint performance is expected.
Since the rotating tool stirs the joint interface, clean surfaces are created and
joined together. This is advantageous in that a preparation step necessary for diffusion
bonding is unnecessary. As described above, the application of the friction stir welding
method to the structural steel sheets is expected to be advantageous in a number of
ways. Although the friction stir welding method is widely used for the low-melting
point metal sheets, the method is not widely used for structural steel sheets because
some problems remain, such as prevention of the occurrence of defects during welding
and welding workability such as an increase in welding speed (i.e., the moving speed
of the rotating tool).
[0008] The main cause of the occurrence of defects in the friction stir welding method described
in Patent Literature 2 is the differences in temperature and plastic flow in the thickness
direction of the metal sheets. When the rotating tool is pressed against one side
of the joint between the metal sheets and moved in the welding direction while rotated,
the rotation of a shoulder of the rotating tool causes a sufficient increase in temperature
and sufficient shear stress on the side against which the shoulder is pressed. Large
deformation thereby occurs at high temperature, and plastic flow enough to obtain
a metallurgically welded state by creating clean surfaces in contact with each other
at the joint interface is obtained. However, on the opposite side, since smaller shear
stress is applied at relatively low temperature, a state in which plastic flow enough
to achieve the metallurgically welded state is not obtained is likely to occur.
[0009] When the friction stir welding technique described in Patent Literature 2 is applied
to structural steel sheets, the above state tends to occur when heat input is low
and the welding speed is high because the strength of the structural steel sheets
at high temperature is high, and it is difficult to increase the welding speed while
the occurrence of defects during welding is prevented.
[0010] Patent Literatures 3, 4, and 5 disclose double-sided friction stir welding methods.
In these double-sided friction stir welding methods, shoulders of a pair of opposed
rotating tools are pressed against the top and bottom sides of the joint between metal
sheets to cause large deformation on both sides by a sufficient temperature increase
and sufficient shear stress generated by the rotation of the shoulders. Plastic flow
enough to obtain a welded state is thereby obtained uniformly in the thickness direction
of the metal sheets, and this may allow a high welding speed to be achieved while
the occurrence of defects during welding is prevented. In the techniques described
in Patent Literatures 3, 4, and 5, the shoulders of the pair of opposed rotating tools
are pressed against the top and bottom sides of the metal sheets. However, no consideration
is given to the gap between the shoulders of the pair of rotating tools that is significant
in obtaining a temperature increase and shear stress enough to obtain the welded state.
Citation List
Patent Literature
Summary of Invention
Technical Problem
[0012] The present invention solves the problems in the conventional technologies and allows
double-sided friction stir welding in which the shoulders of a pair of opposed rotating
tools are pressed against the top and bottom sides of the joint between metal sheets
and the rotation of the shoulders causes a sufficient temperature increase and sufficient
shear stress. This causes large deformation on both sides at higher temperature, and
plastic flow enough to achieve a welded state can be obtained uniformly in the thickness
direction of the metal sheets. Accordingly, it is an object of the present invention
to provide a friction stir welding method in which an increase in welding speed is
achieved while the occurrence of defects during welding is prevented and in which
sufficient strength can be obtained and welding workability can be improved. It is
another object to provide a friction stir welding device suitable for the friction
stir welding. In particular, it is an object to provide a friction stir welding method
in which the pair of opposed rotating tools have a strictly examined gap between their
shoulders that is significant in obtaining a temperature increase and shear stress
enough to achieve a welded state and to provide a friction stir welding device that
embodies the method.
Solution to problem
[0013] The present inventors have conducted extensive studies to solve the foregoing problems
and obtained the following findings (a) to (e).
- (a) In double-sided friction stir welding, to obtain distributions of temperature
increase and shear stress enough to obtain a welded state uniformly in the thickness
direction of metal sheets for the purpose of achieving a high welding speed while
the occurrence of defects during welding is prevented, it is necessary to strictly
control the gap between the shoulders of the pair of opposed rotating tools. In particular,
when a tilt angle is given to the pair of rotating tools, controlling the diameter
and tilt angle of the shoulders of the rotating tools in addition to the thickness
of the metal sheets is effective.
- (b) When the rotation direction of one of the pair of opposed rotating tools on the
top side is the same as the rotation direction of the other one on the bottom side,
the relative speed of one of the rotating tools with respect to the other one is zero.
Therefore, as the plastic flow in the metal sheets at the gap between the shoulders
of the rotating tools approaches a uniform state, plastic deformation decreases, and
heat generation due to the plastic deformation of the metal sheets is not obtained,
so that a good welded state cannot be obtained. In order that a temperature increase
and shear stress enough to achieve a good welded state are obtained uniformly in the
thickness direction of the metal sheets, it is necessary that the rotation direction
of one of the pair of rotating tools be opposite to the rotation direction of the
other one.
- (c) By strictly controlling the gap between the tips of pins of the pair of opposed
rotating tools, the temperature increase and the shear stress can be obtained uniformly
in the thickness direction of the metal sheets, so that the welding speed can be increased
while the occurrence of defects during welding is prevented. Moreover, by adjusting
the thickness of the metal sheets and the diameter of the shoulders of the rotating
tools, the effect becomes significant.
- (d) By strictly controlling the diameter of the shoulders of the pair of opposed rotating
tools, the temperature increase and the shear stress can be obtained uniformly in
the thickness direction of the metal sheets, and the welding speed can be increased
while the occurrence of defects during welding is prevented. In particular, by limiting
the diameter of the shoulders in relation to the thickness of the metal sheets, the
effect obtained becomes significant.
- (e) When the numbers of revolutions of the pair of opposed rotating tools are the
same and the ratio of the welding speed to the number of revolutions is strictly controlled,
the temperature increase and the shear stress can be obtained uniformly in the thickness
direction of the metal sheets, so that the welding speed can be increased while the
occurrence of defects during welding is prevented. In particular, by limiting the
ratio of the welding speed to the number of revolutions in relation to the gap between
the shoulders of the rotating tools, the diameter of the shoulders, and the thickness
of the metal sheets, the effect obtained becomes significant.
[0014] The present invention is based on these findings.
[0015] Accordingly, the present invention provides a double-sided friction stir welding
method including: disposing a pair of opposed rotating tools on top and bottom sides
of a butt joint or a lap joint, which is a joint between two metal sheets; moving
the pair of rotating tools along the butt joint or the lap joint in a welding direction
while the pair of rotating tools are rotated to thereby soften a portion of the metal
sheets by heat of friction between the rotating tools and the metal sheets; and stirring
the softened portion with the rotating tools to generate plastic flow to thereby join
the metal sheets together,
wherein each of the pair of rotating tools used includes a shoulder and a pin that
is disposed on the shoulder and shares a rotation axis with the shoulder, at least
the shoulder and the pin being formed of a material harder than the metal sheets,
wherein, with the metal sheets fixed by a holding unit, the pair of rotating tools
are pressed against the top and bottom sides of the metal sheets and moved in the
welding direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the thickness (mm) of each of the metal sheets when the metal sheets are
butted or is the total thickness (mm) of the metal sheets when the metal sheets are
lapped, and D is the diameter (mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the thickness t (mm) of each of the
metal sheets when the metal sheets are butted or the total thickness t (mm) of the
metal sheets when the metal sheets are lapped satisfy
wherein the gap g, the diameter D (mm) of the shoulders of the rotating tools, and
the thickness t (mm) of each of the metal sheets when the metal sheets are butted
or the total thickness t (mm) of the metal sheets when the metal sheets are lapped
satisfy
wherein the pair of rotating tools are rotated in opposite directions to perform
friction stir welding, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the thickness
t (mm) of each of the metal sheets when the metal sheets are butted or the total thickness
t (mm) of the metal sheets when the metal sheets are lapped satisfy
[0016] The present invention also provides a double-sided friction stir welding device including
a pair of opposed rotating tools that are disposed on top and bottom sides of a butt
joint or a lap joint which is a joint between two metal sheets, the pair of opposed
rotating tools being moved along the butt joint or the lap joint in a welding direction
while rotated to thereby soften a portion of the metal sheets by heat of friction
between the rotating tools and the metal sheets, the softened portion being stirred
with the rotating tools to generate plastic flow to thereby join the metal sheets
together,
wherein each of the rotating tools includes a shoulder and a pin that is disposed
on the shoulder and shares a rotation axis with the shoulder, at least the shoulder
and the pin being formed of a material harder than the metal sheets,
wherein the double-sided friction stir welding device further includes a holding unit
that fixes the metal plates when the pair of rotating tools are moved in the welding
direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the thickness (mm) of each of the metal sheets when the metal sheets are
butted or is the total thickness (mm) of the metal sheets when the metal sheets are
lapped, and D is the diameter (mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the thickness t (mm) of each of the
metal sheets when the metal sheets are butted or the total thickness t (mm) of the
metal sheets when the metal sheets are lapped satisfy
wherein the gap g, the diameter D (mm) of the shoulders of the rotating tools, and
the thickness t (mm) of each of the metal sheets when the metal sheets are butted
or the total thickness t (mm) of the metal sheets when the metal sheets are lapped
satisfy
wherein the double-sided friction stir welding device further includes a rotation
driving unit that rotates the pair of rotating tools in opposite directions, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the thickness
t (mm) of each of the metal sheets when the metal sheets are butted or the total thickness
t (mm) of the metal sheets when the metal sheets are lapped satisfy
Advantageous Effects of Invention
[0017] In the present invention, when double-sided friction stir welding is performed, the
shoulders of the pair of opposed rotating tools are pressed against the top and bottom
sides of the joint of the metal sheets, and the rotation of the shoulders causes a
sufficient temperature increase and sufficient shear stress. This causes large deformation
on both sides at higher temperature, and plastic flow is generated uniformly in the
thickness direction of the metal sheets, so that a good welded state can be obtained.
Therefore, the welding speed can be increased while the occurrence of defects during
welding is prevented. Sufficient strength can be obtained, and welding workability
can be improved, so that an industrially significant effect is obtained.
Brief Description of Drawings
[0018]
[Fig. 1] Fig. 1 shows schematic perspective views of examples of the arrangement of
rotating tools and metal sheets in the present invention, (1) showing the case of
a butt joint, (2) showing the case of a lap joint.
[Fig. 2] Fig. 2(1) is a plan view of one of the rotating tools and the metal sheets
in Fig. 1, and Fig. 2(2) is a cross-sectional view taken along arrow A-A.
[Fig. 3] Fig. 3 shows cross-sectional views illustrating the cross-sectional dimensions
of rotating tools used in Examples.
[Fig. 4] Fig. 4 shows the relation between the axial load between opposed rotating
tools and the gap between shoulders of the opposed rotating tools.
[Fig. 5] Fig. 5 shows the relation between the axial load and the ratio of the welding
speed to the number of revolutions.
Description of Embodiments
[0019] In the present invention, two metal sheets are butted or lapped together, and a pair
of rotating tools are disposed on the top and bottom sides of the butt joint or the
lap joint. Then double-sided friction stir welding is performed.
[0020] Referring to Figs. 1 and 2, double-sided friction stir welding of a butt joint will
be described in detail.
[0021] As shown in Fig. 1, a pair of rotating tools 1 and 8 opposed to each other are disposed
on the top and bottom sides of two butted metal sheets 3. The rotating tools 1 and
8 are inserted into an unwelded joint 12 from both the top and bottom sides of the
metal sheets 3 and moved in a welding direction while rotated. An arrow P in Fig.
1 represents the moving direction of the rotating tools 1 and 8 (i.e., a welding direction).
An arrow Q represents the rotation direction of the rotating tool 1 disposed on the
top side, and an arrow R represents the rotation direction of the rotating tool 8
disposed on the bottom side.
[0022] The pair of opposed rotating tools 1 and 8 are rotated to generate frictional heat
to thereby soften a portion of the metal sheets 3, and the softened portion is stirred
with the pair of rotating tools 1 and 8 to generate plastic flow to thereby join the
metal sheets 3 together. The thus-obtained welded joint 4 is formed linearly in the
moving direction of the rotating tools 1 and 8. A straight line 7 (hereinafter referred
to as a joint center line) extending from the unwelded joint 12 in Fig. 1 through
the center of the welded joint 4 in its width direction coincides with the trajectory
of the rotating tools 1 and 8 moving in the direction of the arrow P (see Fig. 2(1)).
[0023] The two metal sheets 3 are held by a holding unit (not shown) when the rotating tools
1 and 8 are moved along the joint center line 7, and the metal sheets 3 are thereby
fixed at prescribed positions. No particular limitation is imposed on the structure
of the holding unit, so long as the holding unit used can prevent changes in the positions
of the metal sheets 3 during movement of the rotating tools 1 and 8.
[0024] The tip of a pin 6 of the top side rotating tool 1 and the tip of a pin 10 of the
bottom side rotating tool 8 do not abut against each other, and a gap g (mm) is present
therebetween as shown in Fig. 2(2). A gap G (mm) is formed between stepped portions
5 and 9 (hereinafter referred to as shoulders) generated by the difference between
the diameter D (mm) of the rotating tools 1 and 8 and the diameter a (mm) of the tips
of the pins 6 and 10.
[0025] As viewed from the top side, the bottom side rotating tool 8 is rotated in a direction
(a direction of arrow R) opposite to the rotation direction of the top side rotating
tool 1 (i.e., the direction of arrow Q). For example, as shown in Fig. 2(1) that is
a plan view when the metal sheets 3 are viewed from the top side, when the rotating
tool 1 is rotated clockwise, the rotating tool 8 is rotated counterclockwise. Although
not illustrated, when the rotating tool 1 is rotated counterclockwise, the rotating
tool 8 is rotated clockwise.
[0026] As described above, the gap g is formed between the tip of the pin 6 of the rotating
tool 1 and the tip of the pin 10 of the rotating tool 8, and the gap G is formed between
the shoulder 5 of the rotating tool 1 and the shoulder 9 of the rotating tool 8. Moreover,
the rotating tool 1 and the rotating tool 8 are rotated in opposite directions. Since
a sufficient temperature increase and sufficient shear stress are applied from both
sides, the differences in temperature and plastic flow that are generated in the thickness
direction of the metal sheets 3 at the welded joint 4 are reduced, and a uniform welded
state can be obtained. Since poor plastic flow that occurs locally in the welded joint
4 can be eliminated, welding defects can be advantageously eliminated. Therefore,
sufficient strength can be obtained, and welding workability, particularly the welding
speed, can be improved.
[0027] The top side rotating tool 1 includes the shoulder 5 and the pin 6 that is disposed
on the shoulder 5 and shares a rotation axis 2 with the shoulder 5. The bottom side
rotating tool 8 includes the shoulder 9 and the pin 10 that is disposed on the shoulder
9 and shares a rotation axis 11 with the shoulder 9. At least the shoulders 5 and
9 and the pins 6 and 10 are formed of a material harder than the metal sheets 3.
[0028] Since the rotation directions Q and R of the opposed rotating tools 1 and 8 on the
top and bottom sides are opposite to each other, rotating torques applied to the metal
sheets 3 by the rotation of the rotating tools 1 and 8 cancel each other. Therefore,
jigs holding the metal sheets 3 can have a simpler structure than those used in a
conventional friction stir welding method in which a rotating tool is pressed against
metal sheets from one side to join the metal sheets together.
[0029] If the rotation directions of the opposed rotating tools 1 and 8 on the top and bottom
sides are the same, the relative speed of the bottom side rotating tool 8 with respect
to the top side rotating tool 1 is zero. Therefore, as the plastic flow in the metal
sheets 3 at the gap between the shoulders 5 and 9 of the rotating tools 1 and 8 approaches
a uniform state, plastic deformation decreases, and heat generation due to the plastic
deformation of the metal sheets 3 is not obtained, so that a good welded state cannot
be achieved.
[0030] In order that a temperature increase and shear stress enough to achieve a good welded
state are obtained uniformly in the thickness direction of the workpieces, the rotation
directions Q and R of the opposed rotating tools 1 and 8 on the top and bottom sides
are opposite to each other.
[0031] In the present invention, adjusting the arrangement of the rotating tools as follows
is effective in improving the service life of the rotating tools, preventing the occurrence
of welding defects, and increasing the welding speed.
[0032] First, the tilt angle α(°) of the top and bottom side rotating tools will be described.
[0033] The rotation axes 2 and 11 of the rotating tools 1 and 8 are tilted at an angle α(°)
with respect to a direction normal to the metal sheets 3, so that the tips of the
pins 6 and 10 are located on a leading side in the welding direction P. In this case,
loads on the rotating tools 1 and 8 are applied thereto as compressive force components
in the direction of the rotation axes 2 and 11. It is necessary that the pair of rotating
tools 1 and 8 be formed of a material harder than the metal sheets 3. When a low-toughness
material such as a ceramic is used, if a force in a bending direction is applied to
the pins 6 and 10, stress is concentrated locally, and the pins 6 and 10 may break.
Therefore, by tiling the rotation axes 2 and 11 of the pair of rotating tools 1 and
8 at the angle α (hereinafter referred to as a tilt angle), the loads on the rotating
tools 1 and 8 are applied as the compressive force components in the rotation axes
2 and 11. In this case, the force in the bending direction can be reduced, and breakage
of the rotating tools 1 and 8 can be avoided.
[0034] The above effect can be obtained when the tilt angle α exceeds 0°. However, if the
tilt angle α exceeds 3°, the top and bottom surfaces of the welded joint are concaved,
and this adversely affects the welded joint strength. Therefore, the upper limit of
the tilt angle α is 3°. Specifically, the tilt angle α is within the range of 0 <
α ≤ 3.
[0035] Next, the gap G (mm) between the shoulders of the top and bottom side rotating tools
will be described.
[0036] In the double-sided friction stir welding, to increase the welding speed while the
occurrence of defects during welding is prevented, it is necessary to strictly control
the gap G between the shoulders 5 and 9 of the pair of rotating tools 1 and 8. The
gap G is important in order that a temperature increase and shear stress enough to
achieve a welded state are obtained uniformly in the thickness direction of the metal
sheets 3.
[0037] The tilt angle α of the top and bottom side rotating tools 1 and 8 is set to 0° <
α ≤ 3°. The gap G (mm) is set within the range of from (0.5 × t) - (0.2 × D × sin
α) to t-(0.2 × D × sin α) inclusive using the diameter D (mm) of the shoulders 5 and
9 of the rotating tools 1 and 8 and the thickness t (mm) of each of the metal sheets
3 when they are butt welded or the total thickness t (mm) of the metal sheets 3 when
they are lap welded. In this case, the shoulders 5 and 9 of the opposed rotating tools
1 and 8 come into intimate contact with or are pressed into the top and bottom sides
of the metal sheets 3. Therefore, the metal sheets 3 are pressed under a sufficient
load from the top and bottom sides by the shoulders 5 and 9 of the rotating tools
1 and 8.
[0038] Fig. 4 shows the gap G between the shoulders of the top and bottom side rotating
tools and an axial load F therebetween. In this case, steel sheets having the thickness,
chemical composition, and tensile strength shown in No. 1 in Table 1 were used. The
butt joint surface had a surface state equivalent to that obtained by milling, and
were a so-called I-type groove with no beveling. Rotating tools having a cross-sectional
shape shown in Fig. 3(1) and formed of tungsten carbide (WC) were disposed on the
top and bottom sides and pressed against the steel sheets at a tilt angle α of 1.5°to
perform friction stir welding with the number of revolutions of the top and bottom
side rotating tools set to 1,000 rpm and the welding speed set to 2 m/min. In this
case, the limited range of the gap G is from 0.74 mm to 1.54 mm inclusive, and the
axial load F can be 10 kN or more when the gap G is 1.54 mm or less. When the metal
sheets are pressed under a sufficient load, friction by the shoulders 5 and 9 of the
rotating tools 1 and 8 and plastic deformation in a shear direction facilitate heat
generation and plastic flow. The plastic flow is thereby facilitated uniformly in
the thickness direction, and a good welded state can be achieved. If the gap G between
the shoulders 5 and 9 of the pair of rotating tools 1 and 8 exceeds t - (0.2 × D ×
sin α), the shoulders 5 and 9 of the rotating tools 1 and 8 cannot press the top and
bottom sides of the metal sheets 3 under a sufficient load, and the above effect cannot
be obtained. If the gap G is less than (0.5 × t) - (0.2 × D × sin α), the top and
bottom sides of the welded joint are concaved, and this adversely affect the welded
joint strength. Therefore, the gap G satisfies
[0039] Next, the gap g (mm) between the tips of the pins of the top and bottom side rotating
tools will be described.
[0040] To obtain a temperature increase and shear stress uniformly in the thickness direction
of the metal sheets 3 to thereby achieve an increase in welding speed while the occurrence
of defects during welding is prevented, strictly controlling the gap g between the
tips of the pins 6 and 10 of the opposed rotating tools 1 and 8 is effective. In particular,
when the ratio (D/T) of the diameter D of the shoulders 5 and 9 of the rotating tools
1 and 8 to the thickness t (mm) of each of the metal sheets 3 (when they are butt
welded) or the total thickness t (mm) of the lapped metal sheets (when they are lap
welded) is small, the frictional heat generated at the shoulders of the top and bottom
side rotating tools is less likely to be transferred in the thickness direction, and
softening of the material by the heat from the shoulders does not proceed, so that
plastic flow is unlikely to occur uniformly in the thickness direction. Since it is
necessary to generate frictional heat and plastic flow necessary and sufficient to
obtain a welded state from the pins, limiting the gap g between the tips of the pins
6 and 10 within the range of from [0.1 - 0.09 × exp{-0.011 × (D/t)
2}] × t to [1 - 0.9 × exp{-0.011 × (D/t)
2}] × t inclusive is effective. As can be seen from this formula, as D/t decreases,
the upper and lower limits of the gap g are controlled at lower values. The gap g
can be adjusted by changing the positions of the top and bottom side rotating tools
or the length b of the pins of the rotating tools.
[0041] A gap g between the tips of the pins 6 and 10 of less than [0.1 - 0.09 × exp{-0.011
× (D/t)
2}] × t is not preferable because the tips of the pins 6 and 10 of the opposed rotating
tools 1 and 8 may come into contact with each other and break. If the gap g exceeds
[1 - 0.9 × exp{-0.011 × (D/t)
2}] × t, the plastic flow and frictional heat are not effectively obtained uniformly
in the thickness direction. Therefore, gap g is [0.1 - 0.09 × exp{-0.011 × (D/t)
2}] × t ≤ g ≤ [1 - 0.9 × exp{-0.011 × (D/t)
2}] × t.
[0042] Next, the diameter D (mm) of the shoulders of the top and bottom side rotating tools
will be described.
[0043] Controlling the diameter D of the shoulders 5 and 9 of the opposed rotating tools
1 and 8 strictly in addition to the gaps G and g described above is effective in obtaining
a temperature increase and shear stress uniformly in the thickness direction of the
metal sheets 3 to thereby achieve an increase in welding speed while the occurrence
of defects during welding is prevented. When the ratio of the diameter D to t is small,
the frictional heat generated at the shoulders of the top and bottom side rotating
tools is less likely to be transferred in the thickness direction, and softening of
the material by the heat from the shoulders does not proceed, so that plastic flow
is unlikely to occur uniformly in the thickness direction. Therefore, in particular,
by limiting the diameter D within the range of from 4 × t to 20 × t inclusive using
the thickness t (mm) of the metal sheets 3, the effect can be obtained.
[0044] If the diameter D is less than 4 × t, plastic flow uniform in the thickness direction
cannot be effectively obtained. A diameter D exceeding 20 × t is not preferable because
the region in which the plastic flow occurs is unnecessarily broad and an excessive
load is applied to the device. Therefore, the diameter D is 4 × t ≤ D ≤ 20 × t. The
thickness t is the thickness t of each of the metal sheets 3 when they are butt welded
and is the total thickness t of the lapped metal sheets 3 when they are lap welded.
[0045] Next, the ratio T/S of the welding speed T (m/min) of the top and bottom side rotating
tools to the number of revolutions S of the rotating tools will be described.
[0046] To obtain a temperature increase and shear stress uniformly in the thickness direction
of the metal sheets 3 to thereby achieve an increase in the welding speed while the
occurrence of defects during welding is prevented, controlling the ratio (T/S) of
the welding speed T (m/min) of the opposed rotating tools 1 and 8 to the number of
revolutions S (rpm) of the rotating tools strictly is effective.
[0047] In double-sided friction stir welding, frictional heat per unit time Q
time increases as the axial load F between the top and bottom side rotating tools, the
diameter D of the shoulders, or the number of revolutions S increases. Therefore,
the following relation may hold.
[0048] By dividing the frictional heat per unit time by the welding speed T (m/min) and
the thickness t (mm), the amount of heat can be standardized by the distances in the
welding direction and the thickness direction.
[0049] As for the axial load F (kN), it is necessary to give consideration to the relation
between the gap G between the shoulders of the top and bottom side rotating tools
and the axial load F as shown in Fig. 4 and to the relation between the welding speed
T and the number of revolutions S of the top and bottom side rotating tools as shown
in Fig. 5.
[0050] Fig. 5 shows the relation between the axial load F (kN) and the welding speed T ×
1000/the number of revolutions S (mm). In this case, steel sheets having the thickness,
chemical composition, and tensile strength shown in No. 1 in Table 1 were used. The
butt joint surface had a surface state equivalent to that obtained by milling, and
were a so-called I-type groove with no beveling. Rotating tools having a cross-sectional
shape shown in Fig. 3(1) and formed of tungsten carbide (WC) were disposed on the
top and bottom sides and pressed against the steel sheets at a tilt angle α of 1.5°to
perform friction stir welding with the gap G between the rotating tools set to 1.0
mm, the number of revolutions of the top and bottom side rotating tools set to 2,000
to 3,000 rpm, and the welding speed set to 4 to 5 m/min. As the welding speed T/the
number of revolutions S increases, the axial load F tends to increase.
[0051] As can be seen from the experimental tendencies shown in Figs. 4 and 5, the axial
load F is represented by the following formula using the welding speed T, the number
of revolutions S of the top and bottom side rotating tools, the welding speed T, the
number of revolutions S, and the sheet thickness t:
[0052] The QJ-t described above is thereby represented as follows.
[0053] Friction stir welding was performed using steel sheets having the thickness, chemical
composition, and tensile strength shown in No. 1 in Table 1. The butt joint surface
had a surface state equivalent to that obtained by milling, and were a so-called I-type
groove with no beveling. Rotating tools having a cross-sectional shape shown in Fig.
3(1) and formed of tungsten carbide (WC) were disposed on the top and bottom sides
and pressed against the steel sheets at a tilt angle α of 1.5°to perform friction
stir welding with the gap G between the shoulders of the rotating tools set to 0.8
to 1.5 mm, the number of revolutions of the top and bottom side rotating tools set
to 400 to 3,000 rpm, and the welding speed set to 1 to 5 m/min. When the right hand
side of the above formula that is proportional to QJ-t satisfies
heat input is sufficient, and a sound joint with no defects is obtained.
[0054] By modifying the above formula,
is obtained. The ratio T/S of the welding speed T (m/min) to the number of revolutions
S (rpm) of the rotating tools is represented using the ratio D/t of the diameter D
(mm) of the shoulders of the top and bottom side rotating tools to the thickness t
(mm) of each of the metal sheets 3 (when they are butted) or the total thickness t
(mm) of the lapped metal sheets 3 (when they are lapped) and the ratio G/t of the
gap G (mm) between the shoulders 5 and 9 of the rotating tools 1 and 8 to the thickness
t (mm) of each of the metal sheets 3 (when they are butted) or the total thickness
t (mm) of the lapped metal sheets 3 (when they are lapped).
[0055] In particular, when the ratio D/t of the diameter D (mm) of the shoulders 5 and 9
of the rotating tools 1 and 8 to the thickness t (mm) of each of the metal sheets
3 (when they are butted) or the total thickness t (mm) of the lapped metal sheets
3 (when they are lapped) is small, i.e., when the frictional heat generated at the
shoulders of the top and bottom side rotating tools is less likely to be transferred
in the thickness direction and softening of the material by the heat from the shoulders
does not proceed, compositional flow is less likely to occur uniformly in the thickness
direction. Alternatively, when the ratio G/t of the gap G between the shoulders 5
and 9 of the rotating tools 1 and 8 to the thickness t (mm) of each of the metal sheets
3 (when they are butted) or the total thickness t (mm) of the lapped metal sheets
3 (when they are lapped) is large, i.e., when the axial load between the top and bottom
side rotating tools is small relative to the thickness t and the frictional heat generated
between the material and the rotating tools is small, compositional flow is less likely
to occur uniformly in the thickness direction. Therefore, limiting the ratio T/S of
the welding speed T of the opposed rotating tools 1 and 8 to the number of revolutions
S to equal to or less than (1/1000) × (D/t) × {34.5 - 32.2 × (G/t) }/{53 - 3.4 × (D/t)}
is effective. Note that the numbers of revolutions S of the opposed rotating tools
1 and 8 are the same.
[0056] The pins of the top and bottom side rotating tools 1 and 8 may be tapered from the
interfaces with the shoulders toward their forward end. The length b of the pins 6
and 10 may be appropriately determined according to the tilt angle α, the gap G, the
gap g, the diameter D, and the thickness t. The diameter a (mm) of the tips of the
pins 6 and 10 may be set as a matter of design change by one skilled in the art.
[0057] Other welding conditions may be set as a matter of design choice by one skilled in
the art. In this manner, the number of revolutions of the opposed rotating tools 1
and 8 can be set within the range of 100 to 5,000 rpm, and the welding speed can be
increased to 1,000 mm/min or higher.
[0058] The metal sheets 3 used in the present invention may be preferably general structural
steel sheets, carbon steel sheets, steel sheets corresponding to, for example, JIS
G 3106 and JIS G 4051, etc. The present invention is advantageously applicable to
high-strength structural steel sheets with a tensile strength of 800 MPa or more.
Even in this case, the welded joint can have a strength of 85% or more and even 90%
or more of the tensile strength of the steel sheets.
Examples
[0059] Steel sheets having the thicknesses, chemical compositions, tensile strengths shown
in Table 1 were used to perform friction stir welding. When the steel sheets were
butt welded, the butt joint surface had a surface state equivalent to that obtained
by milling, and were a so-called I-type groove with no beveling. Rotating tools were
pressed against the butt joint of the steel sheets from both the top and bottom sides
to perform welding. When the steel sheets were lap welded, two steel sheets of the
same type were lapped, and the rotating tools were pressed against the lap joint of
the steel sheets from both the top and bottom sides to perform welding. As for the
rotation directions of the top and bottom side rotating tools, the top side rotating
tool (the rotating tool 1) was rotated clockwise in a plan view when the steel sheets
(the metal sheets 3) were viewed from the top side as shown in Fig. 2(1), and the
top side rotating tool (the rotating tool 8) was rotated counterclockwise. The welding
conditions for the friction stir welding are shown in Table 2. Two types of rotating
tools formed of tungsten carbide (WC) and having cross sectional shapes shown in Figs.
3(1) and (2) were used.
[Table 1]
N0. |
Sheet thickness (mm) |
Chemical composition (% by mass) |
Tensile strength (MPa) |
C |
Si |
Mn |
P |
S |
1 |
1.6 |
0.3 |
0.21 |
0.69 |
0.012 |
0.003 |
1010 |
2 |
2.4 |
0.16 |
0.07 |
0.69 |
0.016 |
0.009 |
425 |
3 |
1.2 |
0.3 |
0.21 |
0.69 |
0.012 |
0.003 |
1012 |
[Table 2]
|
Thickness of test steel sheet (mm) |
Type of joint |
Top side welding tool |
Bottom side welding tool |
Diameter D of shoulders of top and bottom side rotating tools D (mm) |
Tilt angle α of top and bottom side rotating tools (°) |
Gap G between shoulders of top and bottom side rotating tools (mm) |
Gap g between tips of pins of top and bottom side rotating tools (mm) |
Number of revolutions S of rotating tools |
Welding speed T (m/min) |
T/S |
|
Test steel sheet |
Top side (rpm) |
Bottom side (rpm) |
Inventive Example 1 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
0.5 |
1.20 |
0.27 |
2000 |
2000 |
4 |
0.0020 |
Inventive Example 2 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.10 |
0.31 |
1500 |
1500 |
4 |
0.0027 |
Inventive Example 3 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
3.0 |
0.90 |
0.32 |
1500 |
1500 |
4 |
0.0027 |
Inventive Example 4 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.00 |
0.17 |
2500 |
2500 |
5 |
0.0020 |
Inventive Example 5 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.40 |
0.61 |
3000 |
3000 |
3 |
0.0010 |
Inventive Example 6 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
0.90 |
0.11 |
2500 |
2500 |
5 |
0.0020 |
Inventive Example 7 |
1 |
1.6 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
1.5 |
1.30 |
0.25 |
600 |
600 |
3 |
0.0050 |
Inventive Example 8 |
2 |
2.4 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.50 |
0.71 |
3000 |
3000 |
2 |
0.0007 |
Inventive Example 9 |
2 |
2.4 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
1.5 |
2.20 |
1.15 |
2500 |
2500 |
2 |
0.0008 |
Inventive Example 10 |
2 |
2.4 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
1.5 |
1.80 |
0.75 |
2000 |
2000 |
2 |
0.0010 |
Inventive Example 11 |
3 |
1.2 |
Lap |
20φ-0.7L |
20φ-0.7L |
20 |
1.5 |
2.20 |
1.15 |
2500 |
2500 |
2 |
0.0008 |
Inventive Example 12 |
3 |
1.2 |
Lap |
20φ-0.7L |
20φ-0.7L |
20 |
1.5 |
1.80 |
0.75 |
2000 |
2000 |
2 |
0.0010 |
Comparative Example 1 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
3.0 |
1.50 |
0.92 |
3000 |
3000 |
4 |
0.0013 |
Comparative Example 2 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
4.0 |
1.30 |
0.86 |
2000 |
2000 |
4 |
0.0020 |
Comparative Example 3 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
3.0 |
0.65 |
0.07 |
1500 |
1500 |
4 |
0.0027 |
Comparative Example 4 |
1 |
1.6 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.10 |
0.31 |
1000 |
1000 |
4 |
0.0040 |
Comparative Example 5 |
1 |
1.6 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
4.0 |
1.40 |
0.93 |
600 |
600 |
3 |
0.0050 |
Comparative Example 6 |
2 |
2.4 |
Butt |
12φ-0.5L |
12φ-0.5L |
12 |
1.5 |
1.80 |
1.01 |
3000 |
3000 |
2 |
0.0007 |
Comparative Example 7 |
2 |
2.4 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
0.5 |
1.10 |
0.24 |
2500 |
2500 |
2 |
0.0008 |
Comparative Example 8 |
2 |
2.4 |
Butt |
20φ-0.7L |
20φ-0.7L |
20 |
4.0 |
1.20 |
1.32 |
2000 |
2000 |
2 |
0.0010 |
Comparative Example 9 |
3 |
1.2 |
Lap |
20φ-0.7L |
20φ-0.7L |
20 |
0.5 |
1.10 |
0.24 |
2500 |
2500 |
2 |
0.0008 |
Comparative Example 10 |
3 |
1.2 |
Lap |
20φ-0.7L |
20φ-0.7L |
20 |
4.0 |
1.20 |
1.32 |
2000 |
2000 |
2 |
0.0010 |
[0060] Table 3 shows the presence or absence of surface defects in observation of the appearance
of each weld joint, the presence and absence of internal defects in observation of
a cross section of each joint, and the tensile strength of each weld joint obtained.
The tensile strength was determined by cutting a tensile test piece with dimensions
of No. 1 test specimen defined in JIS Z 3121 from the weld joint obtained and subjecting
the test piece to a tensile test.
[Table 3]
|
Surface defects in observation of appearance of joint |
Internal defects in observation of cross section of joint |
Tensile strength (MPa) |
Inventive Example 1 |
No |
No |
1003 |
Inventive Example 2 |
No |
No |
1005 |
Inventive Example 3 |
No |
No |
1006 |
Inventive Example 4 |
No |
No |
1003 |
Inventive Example 5 |
No |
No |
998 |
Inventive Example 6 |
No |
No |
1002 |
Inventive Example 7 |
No |
No |
995 |
Inventive Example 8 |
No |
No |
421 |
Inventive Example 9 |
No |
No |
422 |
Inventive Example 10 |
No |
No |
423 |
Inventive Example 11 |
No |
No |
994 |
Inventive Example 12 |
No |
No |
999 |
Comparative Example 1 |
Yes (unwelded portion) |
Yes |
552 |
Comparative Example 2 |
Yes (concaved) |
Yes |
534 |
Comparative Example 3 |
Yes (concaved) |
No |
772 |
Comparative Example 4 |
Yes (unwelded portion) |
Yes |
631 |
Comparative Example 5 |
Yes (unwelded portion) |
Yes |
589 |
Comparative Example 6 |
Good |
Yes |
281 |
Comparative Example 7 |
Yes (concaved) |
No |
275 |
Comparative Example 8 |
Yes (concaved) |
No |
284 |
Comparative Example 9 |
Yes (concaved) |
No |
687 |
Comparative Example 10 |
Yes (concaved) |
No |
632 |
[0061] As shown in Table 3, in Inventive Examples 1 to 10 of the butt joint and Inventive
Examples 11 and 12 of the lap joint, even when the welding speed was increased to
2 m/min or higher, no surface defects were found in the observation of the appearances
of the joints, and no internal defects were found in the observation of the cross
sections of the joints, so that a sound welded state was found to be obtained. The
joint strength was 95% more of the tensile strength of the steel sheets used as base
materials.
[0062] In Comparative Examples 1 to 7 of the butt joint and Comparative Examples 8 to 10
of the lap joint, surface defects were found in the joint appearance observation,
or internal defects were found in the joint cross section observation. Both were found
in some cases. Therefore, a sound welded state was not obtained. Moreover, the joint
strength was 70% or less of the tensile strength of the steel sheets used as base
materials.
Reference Signs List
[0063]
- 1
- top side rotating tool
- 2
- rotation axis of top side rotating tool
- 3
- metal sheet
- 4
- welded joint
- 5
- shoulder of top side rotating tool
- 6
- pin of top side rotating tool
- 7
- joint center line
- 8
- bottom side rotating tool
- 9
- shoulder of bottom side rotating tool
- 10
- pin of bottom side rotating tool
- 11
- rotation axis of bottom side rotating tool
- 12
- unwelded joint
1. A double-sided friction stir welding method comprising: disposing a pair of opposed
rotating tools on top and bottom sides of a butt joint which is a welded joint between
two metal sheets; moving the pair of rotating tools along the butt joint in a welding
direction while the pair of rotating tools are rotated to thereby soften a portion
of the metal sheets by heat of friction between the rotating tools and the metal sheets;
and stirring the softened portion with the rotating tools to generate plastic flow
to thereby join the metal sheets together,
wherein each of the pair of rotating tools used includes a shoulder and a pin that
is disposed on the shoulder and shares a rotation axis with the shoulder, at least
the shoulder and the pin being formed of a material harder than the metal sheets,
wherein, with the metal sheets fixed by a holding unit, the pair of rotating tools
are pressed against the top and bottom sides of the metal sheets and moved in the
welding direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the thickness (mm) of each of the metal sheets, and D is the diameter
(mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the thickness t (mm) of each of the
metal sheets satisfy
wherein the gap g, the thickness t (mm) of each of the metal sheets, and the diameter
D (mm) of the shoulders of the rotating tools satisfy
wherein the pair of rotating tools are rotated in opposite directions to perform
friction stir welding, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the thickness
t (mm) of each of the metal sheets satisfy
2. A double-sided friction stir welding method comprising: disposing a pair of opposed
rotating tools on top and bottom sides of a lap joint which is a welded joint between
two metal sheets; moving the pair of rotating tools along the lap joint in a welding
direction while the pair of rotating tools are rotated to thereby soften a portion
of the metal sheets by heat of friction between the rotating tools and the metal sheets;
and stirring the softened portion with the rotating tools to generate plastic flow
to thereby join the metal sheets together,
wherein each of the pair of rotating tools used includes a shoulder and a pin that
is disposed on the shoulder and shares a rotation axis with the shoulder, at least
the shoulder and the pin being formed of a material harder than the metal sheets,
wherein, with the metal sheets fixed by a holding unit, the pair of rotating tools
are pressed against the top and bottom sides of the metal sheets and moved in the
welding direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the total thickness (mm) of the lapped metal sheets, and D is the diameter
(mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the total thickness t (mm) of the
metal sheets satisfy
wherein the gap g, the total thickness t (mm) of the metal sheets, and the diameter
D (mm) of the shoulders of the rotating tools satisfy
wherein the pair of rotating tools are rotated in opposite directions to perform
friction stir welding, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the total
thickness t (mm) of the metal sheets satisfy
3. A double-sided friction stir welding device comprising a pair of opposed rotating
tools that are disposed on top and bottom sides of a butt joint which is a welded
joint between two metal sheets, the pair of opposed rotating tools being moved along
the butt joint in a welding direction while rotated to thereby soften a portion of
the metal sheets by heat of friction between the rotating tools and the metal sheets,
the softened portion being stirred with the rotating tools to generate plastic flow
to thereby join the metal sheets together,
wherein each of the rotating tools includes a shoulder and a pin that is disposed
on the shoulder and shares a rotation axis with the shoulder, at least the shoulder
and the pin being formed of a material harder than the metal sheets,
wherein the double-sided friction stir welding device further comprises a holding
unit that fixes the metal plates when the pair of rotating tools are moved in the
welding direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the thickness (mm) of each of the metal sheets, and D is the diameter
(mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the thickness t (mm) of each of the
metal sheets satisfy
wherein the gap g, the thickness t (mm) of each of the metal sheets, and the diameter
D (mm) of the shoulders of the rotating tools satisfy
wherein the double-sided friction stir welding device further comprises a rotation
driving unit that rotates the pair of rotating tools in opposite directions, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the thickness
t (mm) of each of the metal sheets satisfy
4. A double-sided friction stir welding device comprising a pair of opposed rotating
tools that are disposed on top and bottom sides of a lap joint which is a welded joint
between two metal sheets, the pair of opposed rotating tools being moved along the
lap joint in a welding direction while rotated to thereby soften a portion of the
metal sheets by heat of friction between the rotating tools and the metal sheets,
the softened portion being stirred with the rotating tools to generate plastic flow
to thereby join the metal sheets together,
wherein each of the rotating tools includes a shoulder and a pin that is disposed
on the shoulder and shares a rotation axis with the shoulder, at least the shoulder
and the pin being formed of a material harder than the metal sheets,
wherein the double-sided friction stir welding device further comprises a holding
unit that fixes the metal plates when the pair of rotating tools are moved in the
welding direction while rotated,
wherein the rotation axes of the pair of rotating tools are tilted at a tilt angle
α (°) with respect to a direction normal to the metal sheets such that tips of the
pins are located on a leading side in the welding direction, and the tilt angle α
satisfies
wherein a gap G (mm) between the shoulders that is created by forming a gap g (mm)
between the tips of the pins of the pair of rotating tools satisfies
where t is the total thickness (mm) of the lapped metal sheets, and D is the diameter
(mm) of the shoulders of the rotating tools,
wherein the diameter D (mm) of the shoulders and the total thickness t (mm) of the
metal sheets satisfy
wherein the gap g, the total thickness t (mm) of the metal sheets, and the diameter
D (mm) of the shoulders of the rotating tools satisfy
wherein the double-sided friction stir welding device further comprises a rotation
driving unit that rotates the pair of rotating tools in opposite directions, and
wherein the numbers of revolutions S (rpm) of the pair of rotating tools rotated in
the opposite directions are the same, and the ratio T/S of a welding speed T (m/min)
of the rotating tools to the number of revolutions S of the rotating tools, the gap
G (mm) between the shoulders, the diameter D (mm) of the shoulders, and the total
thickness t (mm) of the metal sheets satisfy